I've found a GPS to be very useful in geotagging pictures, such as these pictures I've taken of the Austin area while on a bike over the years.

The GPS is keeping track of my path and has the time saved with each data point, and my camera stamps each picture with the time, so it was just a matter of some perl to calculate the coordinates of each picture and add it to the map. I was even able to go back to old pictures and geotag them because I had GPS data for my older rides before I started doing this.

Really, the biggest problem is the clock drifting on the camera -- the GPS clock is always right on, but the camera clock isn't always right. But if I take a picture of the GPS's clock during each ride, then I can adjust the time stamps on the pictures appropriately and then tag each picture accurately.

If you take a picture of each sample's source and have the GPS recording at the time, you should be able to work out the locations later. The stuff I wrote to do this is probably a whole lot more complex than is needed, but there is software out there that will do this for you.

As for what GPS to use for all of this, I used an Edge 305 and 705, and I've also used the tracks recorded by my car's Nuvi GPS to do exactly the same thing with my family vacations. I imagine any GPS that keeps your track ought to work.